There is a real need for population control in several species of domestic and wild animals. Methods such as surgical sterilization, or more drastically, culling are generally not acceptable or even allowable in most countries. For example, in Canada the culling or harvesting of seals was prohibited in the early 1980's, resulting in an increased seal population from 10,000 in 1978 to approximately 45,000 at present. Increases in harp seal populations have been much greater. Unfortunately, the increasing population of seals is eating away at the diminishing fish stocks which poses serious problems for the fishing industry. The seals also contain parasites such as seal worms that they pass on to the fish. In 1986 it was estimated that the cost to the fishing industry of removing seal worms from fish by hand was upwards of $30 million a year. Therefore, it is highly desirable to develop an effective form of contraception in mammals, such as seals, in order to effectively control the population growth of certain mammals.
One form of contraception for mammals has involved immunocontraception using glycoproteins isolated from the zona pellucida, a covering which surrounds oocytes. The zona pellucida glycoproteins (hereafter referred to as ZP) provide an attachment site for sperm. Immunocontraception with ZP results in the production of antibodies to ZP which cause (a) an alteration of the nature of the ZP membrane of ova, thereby inhibiting sperm entry, (b) an inhibition of implantation of fertilized ova into the uterus and (c) decreased ovarian follicular differentiation (Henderson, C. J., M. J. Hulme and R. J. Aitken. 1988. Contraceptive potential of antibodies to the zona pellucida. J. Reprod. Fert. 83: 325-343). Immunocontraception has been induced with both zona pellucida glycoproteins and epitopes of these glycoproteins which have been sequenced, synthesized and coupled to carrier proteins (Millar, S. E., S. M . Chamow, A. W. Baur, C. Oliver, F. Robey and J. Dean. 1989. Vaccination with a synthetic zona pellucida peptide produces long-term contraception in female mice. Science 246: 935-938). The use of ZP glycoproteins for immunocontraception has several advantages over other contraception methods. Firstly, ZP glycoproteins are unique to the female reproductive system and therefore, anti-ZP antibodies likely have little or no effect on other tissues. Secondly, the infertility caused by anti-ZP antibodies is reversible, although, hyper- immunization may cause permanent sterility.
Previous studies have shown that in order to effect immunocontraception, multiple injections of ZP were necessary (Kirkpatrick, J. F., I. K. M. Liu and J. W. Turner. 1990. Remotely-delivered immunocontraception in feral horses. Wildl. Soc. Bull. 18: 326-330). Multiple injections are clearly not practical for wild populations as it entails recapturing the same wild animal each time an injection is necessary. Multiple injections are also cumbersome in any situation.
Therefore, it is desirable to develop an immunocontraceptive vaccine which would be effective for long periods following a single injection in an efficient delivery system.
Liposomes have been used to carry drugs to sites of inflammation and infection or in some cases tumours. Liposomes are microscopic spheres composed of either a single or multiple concentric bilayer sheets, and range in size from a nanometer to several micrometers in diameter. These bilayer sheets can be formed from a wide variety of phospholipids in varied formulations. Cholesterol can be included in the bilayer in order to increase the bilayer strength and reduce the leakage of materials encapsulated within the entrapped aqueous interior. A vast array of compounds can be associated with liposomes, including small molecules, drugs, proteins, and nucleic acids. Liposome-associated compounds can be encapsulated within the aqueous interior of the liposome (i.e. between the bilayer sheets), integrated into the bilayer, or adsorbed or attached to the bilayer surface. The location depends upon the properties of the associating compounds as well as the procedures used for the formation of the liposome.
A liposome based vaccine system has been described for immunization against human malaria (Fries et al, 1992. Liposomal Malaria Vaccine in humans: A safe and potent adjuvant strategy. Proc. Natl. Acad. Sci. USA. 89: 358-362). In this system, a recombinant malaria protein derived from Plasmodium falcioarum was encapsulated into liposomes and injected into male volunteers. However the results indicate that at least three injections of the vaccine were required in order to produce an elevated antibody response.